Binding, Sensing, And Transporting Anions with Pnictogen Bonds: The Case of Organoantimony Lewis Acids

Motivated by the discovery of main group Lewis acids that could compete or possibly outperform the ubiquitous organoboranes, several groups, including ours, have engaged in the chemistry of Lewis acidic organoantimony compounds as new platforms for anion capture, sensing, and transport. Principal to this approach are the intrinsically elevated Lewis acidic properties of antimony, which greatly favor the addition of halide anions to this group 15 element. The introduction of organic substituents to the antimony center and its oxidation from the + III to the + V state provide for tunable Lewis acidity and a breadth of applications in supramolecular chemistry and catalysis. The performances of these antimony-based Lewis acids in the domain of anion sensing in aqueous media illustrate the favorable attributes of antimony as a central element. At the same time, recent advances in anion binding catalysis and anion transport across phospholipid membranes speak to the numerous opportunities that lie ahead in the chemistry of these unique main group compounds.


■ INTRODUCTION
Lewis acidic main group compounds have become ubiquitous in both organic and organometallic chemistry as illustrated by the rise perfluorinated triarylboranes 1 and their prevalence in numerous applications. 2Even if much less reactive than their boron halide counterparts, these organoboranes are mostly limited to use in organic media and often readily decompose in aqueous environments.These water compatibility issues can be resolved, at least in part, by using aryl groups like mesityls to sterically protect the boron center against water. 3Using this strategy, several groups, including ours, have prepared watercompatible boranes 4 whose solubilities can be enhanced by the introduction of peripheral cationic moieties. 5These cationic groups can also increase the anion affinity of these boranes, a feature that we have exploited for the sensing of toxic anions such as fluoride and cyanide in aqueous solutions. 6While these boranes can be used to detect parts-per-million (ppm) concentrations of these anions, the binding event is typically accompanied by a fluorescence "turn-off" response 7 thereby restricting their analytical practicality as molecular sensors.These boranes also tend to decompose upon standing in aqueous media over time, and their vulnerability to oxidation 8 challenges their longevity under aerobic conditions, especially in the presence of reactive oxygen species.
However, as documented in the early work of Gutmann, Lewis acidity also emerges as a prevalent property of Sb(V) halides.Using the activation of the C−Cl bond of trityl chloride in MeCN as an indicator, Gutmann demonstrated that SbCl 5 stands near the apex of his chloride ion affinity scale, second only to InCl 3 (Scheme 1). 9 In another series of seminal contributions, Olah, 10 and later Gillespie, 11 showed that SbF 5 could be used for the activation of C−F bonds in organic compounds (Scheme 1) or for the generation of "superacids". 12As seen in boron chemistry, early reports also suggested that organoantimony derivatives retain some of the Lewis acidic characteristics of their halide analogs while displaying much greater tolerance to ambient air and water. 13ncouraged by these precedents, we posited that such compounds may be uniquely suited for anion binding in a variety of competitive media.
Building off these seminal studies, we have undertaken extensive investigations into the properties and applications of organoantimony compounds in our laboratory.In this Perspective, we will review these efforts while spotlighting other notable works also focused on the development of Lewis acidic organoantimony compounds as new anion binding and trafficking platforms.In doing so, we will first discuss the structural and electronic factors that grant antimony its Lewis acidity and show how these basic principles have guided the design of organoantimony-based anion binding units and sensors.We will also highlight the arising utilities of organoantimony compounds for anion-binding catalysis and as transporters of anions across biological membranes.Finally, because this Perspective will only discuss organic derivatives of antimony, we direct the reader's attention to our adjacent work employing antimony as a coordination non-innocent ligand for late transition metal centers, in which the antimony center is also capable of anion binding 14 and sensing. 15ORIGIN OF LEWIS ACIDITY IN SIMPLE ANTIMONY

COMPOUNDS AND RELATED CONCEPTS
The Case of Antimony(III) Derivatives.To understand the origin of the elevated Lewis acidity of antimony compounds compared to lighter group 15 elements, or pnictogens (Pn), we look back to the experimental work of Gutmann, 16 who interrogated the conversion of pnictogen trichlorides (PnCl 3 ) into the corresponding tetrachloropnictate complexes. 9These pioneering studies showed that the chloride affinity of SbCl 3 exceeds that of AsCl 3 and far exceeds that of PCl 3 in MeCN. 9These experimental results are corroborated by the computational work of Bickelhaupt and co-workers who applied the activation strain model to clarify the origin of this simple trend. 17This computational approach, as summarized in Figure 1, 18 considers the formation of a complex based on: (i) the deformation of the interacting partners to geometries that match those in the final complexes; (ii) the formation of the final complex by association of the deformed partners.These two steps give rise to two energy terms, referred to as the strain energy (ΔE strain ) and the interaction energy (ΔE int ), respectively.Furthermore, energy decomposition analysis (EDA) schemes can be used to shed light on how electrostatic forces (ΔV elstat ), orbital-based interactions (ΔE oi ), and Pauli repulsions (ΔE Pauli ) influence ΔE int (Table 1).
First, the ΔE strain needed to distort the Pn(III) center from its ground state geometry to that adopted in the tetrachloropnictate anion is the lowest for the antimony derivative.This lower value can be explained by the Cl−Sb−Cl angles in SbCl 3 which are close to 90°, as in the corresponding antimonate complex.A slight decrease is also seen in ΔE Pauli , reflecting the larger size of antimony and its ability to more easily accommodate an extra ligand.A more important factor is the electrostatic term (ΔV elstat ).The magnitude of this term notably increases down the group as the electropositivity and polarizability of the Pn center also increase.It follows that [SbCl 4 ] − is the most electrostatically stabilized complex of the series.The ΔE oi term is not to be ignored though.Because of the increasing diffuseness and radial nodes of its valence orbitals, the covalency of the newly formed Pn-Cl bond decreases going down the group.Inspection of the values in Table 1 shows that this decrease is moderate with the ΔE oi of the antimony system being only marginally more positive than that for arsenic.Altogether, out of the series considered, SbCl 3 binds to chloride most exothermically.When compared to the arsenic system, the superior Lewis acidity of SbCl 3 can be seen as resulting mostly from the more negative ΔV elstat term, with all other terms offsetting one another (Table 1).
The Case of Antimony(V) Derivatives.The early work of Gutmann summarized in Scheme 1 showed that antimony-(V) derivatives are more Lewis acidic than their trivalent counterparts. 19The same study found that SbCl 5 is more chloridophilic than PCl 5 by nearly 5 orders of magnitude, 9 again suggesting that Lewis acidity increases as the group is descended.This conclusion is supported by several computational studies which show that the computed gas-phase fluoride ion affinities (FIAs) of the pnictogen pentafluorides increase from PF 5 to SbF 5 (Table 2). 20recent activation-strain and EDA study investigated fluoride anion binding by Pn(V) derivatives.21 The data in Table 3, compiled for the PnF 5 species, show that the ΔE oi and ΔV elstat terms become less negative as the group descends because of the lengthening of the Pn-F linkage and the increased diffuseness of the Pn orbitals.The decrease of these stabilizing interactions is compensated for by the Pauli repulsions which become lower as the size of Pn increases.Owing to the balancing of these opposite influences, ΔE int shows little variation down the group. Ths, through the lens of the activation-strain model, the high Lewis acidity of antimony originates from the lower ΔE strain term, which reflects the greater flexibility of SbF 5 .With longer and more diffuse Pn-F σ bonds, the fluoride ligands acquire greater freedom of motion and stand further from one another, allowing the SbF 5 unit to more easily accommodate the C 4v geometry it displays in [SbF 6 ] − .
Is a group 15 Lewis acid the same thing as a pnictogen bond donor?We note that donor−acceptor      interactions involving Pn derivatives as acceptors have been known for several decades and only recently referred to as "pnictogen bonds" (PnBs). 22Akin to the halogen bond 23 and the chalcogen bond, 24 PnBs are often described as resulting from the interaction of a donor (D) with a Pn-centered electrophilic "σ hole", a region of positive electrostatic potential opposite to one of the primary bonds (Figure 2). 22,25In this light, the PnB is often regarded as a "noncovalent" interaction.While such a description may be fitting of cases where the interaction is weak, it presents the danger of oversimplifying the nature of PnBs.Namely, this characterization ignores the contribution of orbital-based interactions, like those identified in the above-mentioned work of Bickelhaupt. 18Such orbital-based interactions were discussed in no uncertain terms in the early work of Alcock 26 who in 1972 identified the ability of main group element-centered σ* orbitals to accept electron density from filled donor orbitals (Figure 2).This concept was revisited by Norman in the specific case of pnictogen derivatives in 1994. 27This notion is also prevalent in the seminal work of Scheiner from 2011 on the weakly associated H 3 N → PH 3 adduct in which the nitrogen lone pair aligns with one of the primary P−H bonds and transfers some of its electron density into the corresponding P−H σ* lobe. 28The importance of such orbital or charge transfer interactions will depend on the nature of both the donor and the Pn acceptor.It can be weak as in the H 3 N → PH 3 adduct or strong as in X − → PnX 3 adducts, in which the newly formed X − → Pn bond displays considerable covalency.It follows that the "non-covalent" adjective as a descriptor of PnBs loses its legitimacy. 18PnBs are simply examples of dative bonds, 29 connecting a donor to a group 15 acceptor ([Pn]), stabilized through the interplay of Coulombic and covalent or orbital-based interactions with possible contribution from dispersion interactions.We therefore consider that any D → [Pn] complex can also be described as a Lewis adduct and a PnB donor as a Lewis acid.
■ ANION COMPLEXATION CHEMISTRY Antimony(III) Compounds.The Lewis acidity of antimony persists despite the passivating effect of an aryl appendage.Indeed, simple arylantimony(III) dihalide species are sufficiently Lewis acidic to form isolable antimonate complexes.Such is the case of PhSbCl 2 (1), which exists in the solid-state as a weak contact aggregate 30 and reacts with chloride to form the bridged dianion ([1-(μ-Cl) 2 -1] 2− , Scheme 2). 31Each antimony(III) center of this centrosymmetric dimer adopts a local square pyramidal geometry, with pairs of chloride ligands in trans positions to each other.The disposition of these two chloride ligands can be viewed as resulting from their addition to the sites featuring the deepest σ holes and the most energetically accessible σ* orbitals.Moreover, treating 1 with 2 equiv of chloride gives rise to the monomeric, square pyramidal-shaped dianion [1-Cl 2 ] 2− , further indicating that both binding sites are accessible to an incoming chloride.31a Similarly, diarylantimony(III) halide species have also been shown to display notable Lewis acidity, and are also capable of Lewis base recognition at a binding site that is also trans to the halide ligand. 32For example, the monomeric Ph 2 SbCl (2) 33 also reacts with chloride anions to form [Ph 2 SbCl 2 ] − ([2-Cl] − , Scheme 2), which takes on a seesaw geometry with the two chloride ligands trans to each other.31a, 34 To the best of our knowledge, the dianionic [2-Cl 2 ] 2− has never been isolated, pointing to the inaccessibility of the σ hole/σ*-orbital trans to a phenyl group.
Complete arylation of the antimony(III) center typically limits the chemistry of triarylstibines to use as L type ligands in coordination chemistry. 35However, the introduction of electron-withdrawing aryl groups can elicit PnB donor properties at the antimony center.Such is the case of (C 6 F 5 ) 3 Sb (3), which forms a structurally characterized adduct with Ph 3 PO. 19The Lewis acidic behavior of 3 also manifests in its affinity for chloride anions, as reported by Matile. 36   shows no measurable affinity for the ionic guest, pointing to the crucial role played by the number of pentafluorophenyl substituents appended to the antimony center (Figure 3).
Recent work by the group of Beer further illustrates the benefits resulting from introducing electron-withdrawing units on Ar 3 Sb systems (Figure 3). 37Indeed, 6−12 bind chloride in d 8 -THF as indicated by 1 H NMR spectroscopy.Chloride binding peaks in the case of 12, which bears the most electron deficient aryl rings of the compounds studied.Moreover, the chloride affinity of 12 was markedly higher than its affinity for Br − , I − , and NO 3 − , and more basic anions such as OAc − , OCN − , and NO 2 − were found to bind more weakly than Cl − .We will note in passing a possible disconnect in the magnitude of the chloride binding constants measured by the Matile and Beer groups since the C 6 F 5 and the 3,5-(CF 3 ) 2 C 6 H 3 aryl groups have similar electron-withdrawing properties when appended to main group Lewis acids. 19It is also worth noting that Ph 3 Sb (13) showed no measurable affinity for chloride in d 8 -THF.
Anion binding has also been investigated by the Cozzolino group using oxygen-bridged bidentate distibines 38 such as 14 which displays halide binding constants in CH 2 Cl 2 of 870,000 ± 98,000 M -1 , 16,900 ± 5,000 M −1 , 2,390 ± 1,470 M −1 for chloride, bromide, and iodide, respectively (Figure 4). 39In addition to showing halide anion binding selectivity, 14 also interacts with the cyanide anion and effectively promotes its phase transfer for applications in cyanation chemistry.
Antimony(V) Compounds − Halostiboranes.Harkening back to initial studies on the anion binding chemistry of antimony (Scheme 1), Gutmann found that SbCl 5 possesses a chloridophilicity that is nearly 5 orders of magnitude greater than that of SbCl 3 . 9The increased Lewis acidity observed upon oxidation can be correlated to a deepening of the σ hole at antimony as well as a lowering in the energy of the σ* orbital aligned with an incoming Lewis base, as illustrated in Figure 5. 19 These effects, which have been examined in the special case of 3 and its corresponding tetrachlorocatecholatostiborane (vide infra), 19 should apply to any pair of antimony Lewis acids differentiated by the +III or +V oxidation state of the antimony center.
The Lewis acidity of the Sb(V) compounds can be observed in arylated derivatives such as PhSbCl 4 (15) and Ph 2 SbCl 3 (16), which readily form the corresponding chloroantimonate species Further substitution of the antimony center by aryl substituents can be anticipated to negatively affect its Lewis acidity.These deleterious effects are already evident when analyzing the behavior of Ph 3 SbCl 2 , which to our knowledge, does not associate with chloride ions.On the other hand, Ph 3 SbF 2 (17), which contains smaller and more electronwithdrawing ligands, complexes fluoride in CDCl 3 to form [Ph 3 SbF 3 ] − ([17-F] − , Scheme 4) as verified by 19 F NMR. 42 A stronger complexation was observed in the case of (C 6 F 5 ) 3 SbF 2 (18), leading to the unambiguous characterization of [(C 6 F 5 ) 3 SbF 3 ] − ([18-F] − ) formed by addition of CsF to 18 in MeOH. 43Another strategy to elevate the anion affinity of triaryldihalostiboranes rests on the introduction of hydrogen bonding (HB) groups, as in the case of ( 19), a complex featuring two 2,6-difluoro-N-phenylbenzenamine groups.Our investigation of this derivative shows that it outcompetes Ph 3 SbF 2 for fluoride binding in CDCl 3 at −30 °C. 42nion complexation could also be achieved by pentacoordinate antimony(V) systems in which the antimony center is

Antimony(V) Compounds -Catecholatostiboranes.
Monofunctional Systems.The imposition of geometrical constraints is also an effective way to enhance the Lewis acidity of antimony derivatives.One of the earliest pieces of evidence for these effects was provided in the case of Ph 3 SbO 2 C 6 H 4 (21, Scheme 6) that, based on X-ray analysis, engages a water molecule via formation of an O → Sb bond of 2.51 Å. 13b The formation of such a water adduct is not observed for Ph 3 SbF 2 and Ph 3 SbCl 2 .This contrasting behavior points to the role played by structural constraints imposed by the SbO 2 C 2 fivemembered ring which elevates the Lewis acidity of the antimony(V) center via distortion of its ground state geometry.Another favorable feature of catecholate groups is their minimal steric profile since the coordinating oxygen ligands extend out from the aromatic backbone and thus partly escape its steric hindrance.The formation of [22-Cl][Et 3 NH] by reaction of Ph 3 SbCl 2 with 2,3-naphthalenediol and Et 3 N is another important precedent directly relevant to the topic of this Perspective since it involves the complexation of a halide anion (Scheme 6). 45The formation of [22-Cl] − contrasts with the absence of reactivity of Ph 3 SbCl 2 toward the chloride anion.
In addition to their elevated Lewis acidity, catecholatostiboranes also benefit from their ease of preparation.In addition to metathesis reactions such as that used to form 22, these species can also be obtained through the simple combination of a triarylstibine with an ortho-quinone such as o-chloranil.Owing to its perchlorination, this quinone is a particularly potent oxidant that engages a range of organoantimony compounds, including 13 which is readily converted into Ph 3 SbO 2 C 6 Cl 4 (23, Figure 6) as demonstrated by Holmes in 1987. 45Our reinvestigation of this compound shows that unlike its trivalent precursor, it behaves as a potent Lewis acid, able to bind a fluoride anion when treated with [TBA]-[Ph 3 SiF 2 ] (TBAT).The formation of [23-F][TBA] took place in CH 2 Cl 2 and survived aqueous workup, speaking to the stability of the Sb−F bond. 46The solid-state structure of [23-F] − displays a short Sb−F bond distance of 1.9877(13) Å (Figure 6), only slightly longer than the average Sb−F bond length in [SbF 6 ] − (1.844 Å). 47 As noted at the start of this section, we have explained the increased Lewis acidity observed upon oxidation by a lowering of the σ* orbital and a deepening of the σ hole. 19These effects, combined with the structural predisposition resulting from the presence of a SbO 2 C 2 five-membered ring, may serve to rationalize the anion affinity of molecules such as 23.
Altogether, the conversion of 13 into 23 establishes a simple approach for converting potentially Lewis basic triarylstibines to Lewis acidic stiboranes.We found that even electron poor stibines such as (C 6 F 5 ) 3 Sb (3) and (3,5-(CF 3 ) 2 C 6 H 3 ) 3 Sb (12  could be converted into the corresponding tetrachlorocatecholatostiboranes 24 and 25 (Figure 7). 19As is the case for the parent triphenyl derivative, the antimony-centered LUMO is stabilized by 0.89 and 0.78 eV upon formation of 24 and 25, respectively.A sizable increase in the FIA values (δ(FIA)) was also computed for these three pairs of compounds upon oxidation of the antimony center.The largest change was observed in the case of the parent derivative 13 whose FIA increases by δ(FIA) = 150 kJ•mol −1 upon conversion into 23 versus 111 kJ•mol −1 and 132 kJ•mol −1 for the 3/24 and 12/25 couples, respectively.This strategy has been applied by Matile on other fluorinated triarylstiboranes including (3,4,5-F 3 C 6 H 2 ) 3 Sb (8), which was readily converted into the corresponding tetrachlorocatecholatostiborane 26. 48e should also acknowledge the growing role that perhalogenated catecholate groups are playing in the chemistry of arsenic, 49 silicon, 50 germanium, 51 and phosphorus Lewis acids. 49,52e also aimed to assess the influence of catecholate perchlorination on the Lewis acidity of the antimony center.For example, we decided to compare the anion affinity of 27 45 to that of the perprotiocatecholate analogue 28.We found that 27 displays a high binding constant for the fluoride anion (K(F − ) of 13,500 ± 1,400 M −1 in THF:water (7:3 (v/v)) as established by a UV−vis titration while 28 shows no affinity for the anion under the same conditions (Figure 8). 53The contrasting behaviors of these two compounds underscore how important the electronic features of the catecholate are in governing the anion affinity of these compounds.In this case, perchlorination of the catecholate most likely reduces the donicity of the oxygen atoms, leaving a more exposed and thus more Lewis acidic antimony(V) center.The anionic complex [27-F] − , which has been characterized as a tris-(dimethylamino)sulfonium ([TAS] + ) salt, displays a short Sb−F bond length of 1.973(4) Å, on par with that of the aforementioned [23-F] − . 53In addition to demonstrating the importance of catecholate perchlorination, compound 27 may also benefit from its bicyclic structure.The integration of two antimony-fused five membered rings most likely accentuates the geometrical constraints imposed on the ground state structure, boosting its Lewis acidity. 54

■ BIFUNCTIONAL SYSTEMS
The facility by which tetrachlorocatecholatostiboranes can be prepared prompted us to investigate their introduction in more elaborated scaffolds, including bifunctional ones that would support anion chelation.4b,c,6,55 To explore this possibility, we synthesized the 4,5-bis(diphenylantimony)-9,9-dimethylxanthene (29) and found that it could be easily converted into 30 (Figure 9). 46A structural analysis of 30 shows that the two square-pyramidal stiborane units orient their basal square face inward, thus defining a binding pocket possibly adapted to the capture of small anions.Another favorable feature of this derivative lies in the nature of its LUMO, which spans the two antimony atoms and displays significant σ*(Sb−C/O) parentage with lobes extending toward the center of the binding pocket.
Congruent with these attributes, 30 readily complexes a fluoride anion to afford [30-F] − (Figure 10).A crystal structure of its [TBA] + salt confirms the formation of a fluoride chelate complex supported by two Sb−F bonds of 2.1684(19) Å and 2.1621 (19) Å. 46 This distance shows a measurable elongation compared to the value of 1.9877(13) Å found for the terminal Sb−F bond in [23-F] − , in line with the μ 2 coordination mode of the fluoride anion in [30-F] − .The thermodynamic impact of anion chelation manifests in the acidity constant (pK Sb ) of the antimony center, which is equivalent to the pH at which 50% of the antimony Lewis acid is neutralized by a hydroxide anion.Indeed, the pK Sb of 30 (5.8   Despite its promising attributes, fluoride chelation by 30 positions the bound fluoride above the oxygen of the xanthene backbone, leading to a repulsive O•••F − interaction that likely dampens the anion affinity of the chelator.To circumvent such a situation, we considered the 1,8-triptycenediyl linker as a backbone whose folded structure would lessen repulsive interactions between the chelated anion and the bridgehead C−H unit.Using standard protocols, we generated the distibine 31 which was easily converted into the distiborane 32 (Scheme 7). 56This compound readily complexes a fluoride anion via treatment with TBAT in CH 2 Cl 2 to form [32-F][TBA] (Figure 11). 56Similar to [30-F] − , a crystallographic analysis of this complex again shows the two Sb(V) centers clamping down on the bridging fluoride anion that is held by two short Sb−F bonds of 2.158(2) and 2.251(2) Å.Also, the Sb−Sb separation contracts from an average value of 5.20 Å in 32 to 4.3564(5) Å in [32-F] -.NBO analysis reveals the presence of an lp(F − ) → σ*(C bridgehead −H) donor−acceptor interaction of 5.9 kJ•mol −1 (Figure 11).Though weak, this C−H•••F − bond can be clearly observed by 1 H NMR spectroscopy.The triptycene derivative 32 also has a higher computed FIA value (395 kJ•mol −1 ) than 30 (365 kJ•mol −1 ), suggesting that it is more Lewis acidic .This computational result is corroborated by quantitative fluoride transfer from [30-F] − to 32 as confirmed by 19 F NMR spectroscopy.
Our work on these bidentate systems has also explored 33 and 34, two derivatives based on the o-phenylene backbones and featuring the octafluorophenanthrene-9,10-diolate ligand on each antimony, as opposed to the o-tetrachlorocatecholate ligand found in 30 and 32 (Scheme 8). 57These compounds, which could be accessed using the corresponding distibines following oxidation with 9,10-perfluorophenanthraquinone, differ from one another via the nature of the o-phenylene backbone, which is perfluorinated in the case of 34.Each readily complexes fluoride upon addition of TBAT to form the  12).The chelating binding motifs greatly enhance the FIA compared to the monofunctional 35 (FIA = 327 kJ•mol -1 ), further confirming the efficacy of this strategy for anion binding (Figure 12).Antimony(V) Compounds − Stibonium Cations.Tetraarylstibonium Cations.Historical precedence has shown that the simplest tetraarylstibonium cation [Ph 4 Sb] + ([36] + ) favors the coordination of small, charge-dense anions 58 compared to larger anions. 59Notably, [36] + has long been known to extract aqueous fluoride under biphasic conditions.13a,60 Structural analyses of the halide salts of [36] + indicate that smaller anions may be privileged for tight complexation by the Lewis acidic antimony center. 61Indeed, the solid-state structure of 36-F displays an Sb−F distance of 2.0530(8) Å, 58g which slightly exceeds the sum of the covalent radii of the two elements (R cov (Sb) + R cov (F) = 1.96Å). 62 However, as the halide (X) becomes heavier, the Sb−X bond length increasingly departs from its ideal covalent value, as indicated by the increasing formal shortness ratio (r = observed Sb−X bond length/(R cov (Sb) + R cov (X)), Figure 13). 63This increase in r reflects the greater ionicity of the Sb−X interaction.
Our efforts to explore the upper limit of Lewis acidity in tetraarylstibonium cations has led us to consider perfluorinated derivatives. 65Toward this end, we prepared (C 6 F 5 ) 4 SbCl ([37-Cl) by reaction of SbCl 5 with C 6 F 5 Li. 66In line with the increased Lewis acidity imparted to the antimony center by the electron deficient aryl rings, 37-Cl features an Sb−Cl bond of 2.4509(11) Å, which is much shorter than the values reported for 36-Cl (2.6860(9) Å and 2.7395(10)).58c, 64 The high Lewis acidity of [37]   Figure 13.Structures of halide salts of [36] + with Sb-X bonds that become increasingly ionic, as depicted by their increasing formal shortness ratio (r) values.The r values were calculated from published metrical parameters for 36-F, 58g 36-Cl, 58c,64 36-Br, 58d,59 and 36-I. 61heme 9. Synthesis of [37] + and Fluoride Abstraction Reactions how the antimony center of [37] + privileges the smallest halide anion. 68he anion complexation properties of tetraaryl stibonium centers persist even when near intramolecular Lewis bases. 69uch is the case of [38] + (Figure 14), which forms a strong intramolecular Lewis interaction with an ortho positioned phosphine oxide (Sb−O distance: 2.4315(13) Å). 70 Even if a comparison of the electrostatic potential (ESP) maps of [36]  Exploiting the simplicity by which triarylmethylstibonium cations can be synthesized, we prepared [44] 2+ as a simple bifunctional analogue and isolated this dication as a bis-(triflate) salt ( [44][OTf] 2 , Figure 16).73c The crystal structure of this salt indicates that one of the triflate counterions bridges  12) and 2.9838(13) Å.These distances are significantly shorter than that found in [43][OTf] (3.1518(17) Å), suggesting that the bidentate dication [44] 2+ is a more potent acceptor.
Anion chelation has also been unambiguously characterized in the case of the heteronuclear stibonium-borane cation [46] + . 76Indeed, this cation, which could be easily synthesized by reaction of the corresponding stibinoborane 45 with MeOTf (Figure 17), forms the corresponding fluoride complex  77 the phosphorus analogue of 46-F, shows quantitative transfer of the fluoride anion to [46] + in line with the greater Lewis acidity of the heavier pnictogen.The chelating properties of [46] + are also observed in the complexes that this cation forms with azide and cyanide anions. 78ANION BINDING CATALYSIS Carbon−Halide Bond Activation.Anion binding catalysis has emerged as a popular strategy for the activation of organic substrates whose electrophilic characteristics can be enhanced by the cleavage of a carbon−halide (C−X) bond.This strategy has been successfully implemented using HB donor activators that engage the halide anion and promote the heterolytic dissociation of the C−X bond. 79These advances have prompted the exploration of related approaches in which the HB donor activator is replaced by a main group Lewis acid such as a halogen-80 or chalcogen-bond donor. 81Though more limited, examples also exist in which such reactions have involved PnB donors as activators.Matile and co-workers explored this possibility using stibines 3, 4, and 5, which, as summarized earlier in Figure 3, display an affinity for the chloride anion that scales with the number of pentafluorophenyl groups. 36The contrasting anion binding abilities of these compounds also inform their catalytic activities in reactions involving C−Cl bond activations.An example of such a reaction is provided by the activation of 1-chloroisochroman and the generation of an oxocarbenium intermediate that is readily intercepted by a ketene silyl acetal (Figure 18). 36The yield of this reaction, after 55 h, increases in lockstep with the number of pentafluorophenyl groups on the stibine center, indicating the favorable influence of the Lewis acidity of the pnictogen bond donor.
Additional studies carried out in our laboratory have explored the role played by the redox state of the antimony center in the Ritter-type reaction of diphenylbromomethane (Figure 19). 19This reaction, which necessitates the activation of a C−Br bond, was investigated in CD 3 CN at 40 °C over the course of 24 h using stibine 12 and catecholatostiborane 25 as catalysts.While the reaction proceeded with both catalysts, the conversion obtained with the antimony(V) derivative was markedly higher, as indicated in Figure 19.These results are most simply rationalized by invoking the greater Lewis acidity   of the oxidized antimony center of 25.These results show that the redox state of the antimony atom provides another handle that can be adjusted to augment the catalytic properties of these compounds in reactions that proceed by an anion abstraction step.
Metal−Chloride Bond Activation.The Lewis acid induced heterolysis of metal−halogen (M−X) bonds is a common strategy for the activation of transition metal catalysts. 82While this method typically necessitates the use of strong Lewis acids such as fluorinated boranes, recent results have shown that the M−X bond of softer metals can be activated by hydrogen, 83 halogen, 84 and chalcogen-bond donor functionalities. 85One of our contributions to the development of such approaches has tested whether arylstibine dihalides may be sufficiently chloridophilic to activate M−Cl bonds for application in catalysis.To explore this question, we prepared 48, a ligand containing a phosphine for metal coordination and a dichlorostibine unit as a chloridophilic PnB donor. 86uration with (tht)AuCl produced compound 48-AuCl, allowing us to test the possibility of Au−Cl bond activation by the intramolecularly installed dichorostibine functionality.Addition of Ph 3 P produced the trichloroantimonate-containing zwitterion 49, pointing to the ability of the dichorostibine moiety to indeed participate in Au−Cl bond activation.This conclusion is also supported by the activity of 48-AuCl as a catalyst for the cyclization of N-(prop-2-yn-1-yl)adamantine-1carboxamide.This reaction, which produced the two isomers shown in Figure 20, proceeded to 87% conversion in 33 h when the reaction was carried out in CD 2 Cl 2 with 2 mol % catalyst loading.For comparison, we also synthesized complex 50-AuCl that features a triarylantimony unit as a much less potent PnB donor.This compound proved significantly less active than 48-AuCl supporting the proposal that the dichlorostibine moiety of 48 activates the gold center, as depicted in the working model in Figure 20.
■ ANION SENSING CHEMISTRY Neutral Platforms.As noted in the preceding sections, antimony derivatives display elevated anion affinities, leading us to question whether applications in anion sensing could be developed.Working toward this objective, we considered the case of chlorostibines such as Ph 2 SbCl (2) that readily complex chloride anions to afford the corresponding [Ph 2 SbCl 2 ] − ([2-Cl] − , Scheme 2).Positing that these simple anion binding events could form the basis of a halide recognition system, we decided to target a chlorostibine, in which the antimony(III) atom is incorporated in a conjugated hydrocarbon π-system.Toward this end, we synthesized the stibaindole 51 as a bright yellow solid. 87According to TD-DFT calculations (level of theory: MPW1PW91/Sb: aug-cc-pVTZ-pp, Cl: ECP10MWB, C/H: 6-31g(d)), this compound shows effective conjugation of the π* orbital of the conjugated hydrocarbon backbone and the σ*(Sb−Cl) orbital, which both end up contributing to the LUMO.This compound responds optically to the presence of halide anions including Cl − in MeCN through the loss of its yellow color.This colorimetric response originates from the chloride-induced population of the σ*(Sb−Cl) orbital and the accompanying disruption of the aforementioned σ*−π* conjugation operative in the LUMO (Figure 21).In support of this interpretation, the solid-state structure of [51-Cl] − shows that the chloride anion binds to the antimony center trans to the chloride ligand along a direction perpendicular to the stibaindole system.A titration experiment carried out in MeCN affords a K(Cl − ) of 95,000 ± 5,000 M −1 .This elevated binding constant, which is comparable to that of (C 6 F 5 ) 3 Sb (3), 36 speaks to the high Lewis acidity of 51.This behavior contrasts with that of the phenyl analogue 52, which shows no evidence of chloride binding under the same conditions.These diverging behaviors illustrate the determining role played by the chloride ligand in its ability to lower the energy of the antimony-centered LUMO while also deepening the corresponding σ-hole.
Contemplating the possibility of anion sensing in aqueous environments and bearing in mind the high hydration energy that typifies anions such as fluoride, we also investigated inherently more Lewis acidic antimony(V) compounds.With this in mind, we generated the bright yellow catecholatostiboranes 53 and 54 by oxidation of 52 with o-chloranil and 3,5-ditert-butyl-o-benzoquinone, respectively (Figure 22). 88The origin of the colors displayed by these stiboranes can be traced back to a stibaindole-based HOMO-1−LUMO  In another strategy, we considered analogs of the catecholatostiborane 27 (Figure 8), a compound that displays a high affinity for the fluoride anion in aqueous solutions but lacks any distinct photophysical response in the visible part of the spectrum. 53Aiming to remediate this situation, we decided to replace the tetrachlorocatecholate ligand by a more performant chromophore such as alizarin, 89 which could be easily introduced into 55 using standard protocols (Figure 23). 53 Cationic Platforms.Our earlier work on cationic boronbased anion sensors revealed the determining influence that charge effects can exert over the anion binding at the boron center.4c,6 To build on these precedents, and encouraged by the demonstrated ability of [Ph 4 Sb] + ([36] + ) to sequester the fluoride anion under biphasic conditions, 13a,60 we became eager to investigate the potential of stibonium cations as anion sensors.Since [36] + lacks a read-out photophysical response, we replaced one of the phenyl groups by a 9-anthryl group, leading to the isolation of [56] + (Figure 24). 90This watercompatible stibonium cation is a potent Lewis acid.Because of competitive hydroxide coordination, fluoride binding was studied in a water:DMSO (9:1 (v/v)) solution buffered at pH 4.8.A UV−vis titration experiment carried out under these conditions afforded K(F − ) = 12,000 ± 1,100 M −1 .To our   surprise, fluoride binding by [56] + elicited a fluorescence increase from Φ F = 2.2% to Φ F = 14.1% (Figure 24), enabling the use of this stibonium cation as a fluoride anion fluorescence turn-on sensor compatible with subppm concentrations of the anion.Moreover, we found that this response was selective over other anions like Cl − , enabling fluoride sensing in tap and bottled water samples.
The mechanism of this fluorescence turn-on response was examined by Irle and co-workers via excited-state DFT calculations on [56] + (level of theory: CAM-B3LYP/Sb: CRENBL, C: 6-311+G(d), H: 6-31G). 91Two excited state minima were found in these calculations: an emissive bright state and a nonemissive dark state that lies at a lower energy.In the bright state, the antimony center retains its expected tetrahedral geometry with the HOMO and LUMO corresponding to the π and π* orbitals of the anthryl ligand (Figure 25).The dark state is reached via a distortion of the antimony coordination geometry from tetrahedral to seesaw.This distortion, which lowers the energy of the excited state, also changes the nature of the frontier orbitals.The LUMO is particularly affected as it loses its anthryl π* character and relocates on the antimony atom in the form of a σ*(Sb−C) orbital.This distorted excited state is nonemissive, presumably because of the relatively narrow separation between the two SOMOs.Donation of a fluoride lone pair into the σ*(Sb−C) orbital prevents access to such a dark state in 56-F thus restoring the π−π* emission of the anthracene chromophore.This mechanism also applies to other stibonium cations such as the pyrenyl and perylenyl systems [57] + and [58] + , 92 and the BODIPY derivative [59] + (Figure 26). 93All three cations see their fluorescence readily increase upon fluoride binding at the antimony center.In the case of [57] + , the fluorescence quantum yield increases from Φ F = 0.5% to Φ F = 5.2% upon fluoride binding. 92A proportionally similar response is observed for [58] + , since fluoride binding elicits a fluorescence increase from Φ F = 7.3% to Φ F = 59.2%.Like [56] + , [57] + and [58] + are potent fluoride binders with K(F − ) values of 10,000 ± 800 M −1 and 10,000 ± 500 M −1 for [57] + and [58] + in a water:DMSO solution (9:1 (v/v), pH 4.8, 10 mM pyridine, 10 mM cetyltrimethylammonium bromide).The BODIPY derivative [59] + could only be evaluated in MeCN, a medium in which it displays a K(F − ) greater than 10 7 M −1 and a noticeable fluorescence turn-on response from Φ F = 0.15 to Φ F = 0.30 upon fluoride complexation. 93DFT calculations (level of theory: B3LYP/Sb: aug-cc-pVTZ-PP, B/F: 6-31g(d′), C/ H/N: 6-31g(d)) suggest that the turn-on response of [59] + follows a similar mechanism to that of [56] + , with anion binding into the σ*(Sb−C phenylene ) orbital rescuing the π−π*based emission of the BODIPY fluorophore. 88iming to replace the electron-poor BODIPY chromophore of [59] + by an electron-rich one, we synthesized the carbazole derivative [60] + and found it to be very weakly emissive (Φ F = 0.007 in MeCN) (Figure 27). 94Addition of TBAF in MeCN affords a K(F − ) > 10 7 M −1 , and conversion to 60-F elicits an order-of-magnitude increase in its quantum yield to Φ F = 0.060.Excited state DFT calculations (level of theory: CAM-B3LYP/Sb: aug-cc-pVTZ-PP with CRENBL ECP, C/H/N/F: 6-31+g(d′)) found that this turn-on mechanism differs from that of [56] + .While the HOMO and LUMO of 60-F are

ANTIMONY COMPOUNDS
Beyond the realms of catalysis and optical sensing, anion recognition is also essential to a myriad of biological processes.Chief among these processes is the selective transport of anions across hydrophobic membranes, which is typically facilitated by cell-membrane-embedded proteins that form a selective channel to allow for the passage of anions.In so-called channelopathic diseases, 95 defects in the structure or the expression of these proteins disrupt homeostatic anion transport.Perhaps the best known of these diseases is cystic fibrosis, wherein mutations of the CFTR chloride-bicarbonate anion channel dysregulate its transport activity, leading to a range of harmful consequences. 96Investigation into smallmolecule treatments for channelopathies has been spent developing "carrier-type" anion transporters that are typically water stable, lipophilic, and adorned with an HB donor binding pocket.Owing to these attributes, such compounds readiliy form anion complexes that can diffuse through lipid bilayers, thus allowing for the transmembrane transport of the anionic cargo. 97he parallels existing between HB-based anion transporters and organoantimony anion binders discussed previously have led our group and that of Matile to test whether electron deficient stibines and stibonium cations could also promote anion transport across phospholipid bilayers via a carrier-type mechanism such as that depicted in Figure 28. 98As such, the transport activities of Ph(C 6 F 5 ) 2 Sb (4) were assessed in egg yolk phosphatidylcholine (EYPC) large unilamellar vesicles (LUVs) loaded with the ratiometric pH probe 8-hydropyrene-1,3,6-trisulfonic acid (HPTS) and a buffered solution of NaCl (Figure 29). 99Upon administration of a base pulse to the external solution in which the LUVs are suspended, followed by administration of 4, rapid dissipation of the pH gradient was exhibited via Cl − /OH − exchange, as indicated by changes in the HPTS fluorescence spectrum.
The EC 50 value of this transporter, equal to the concentration of the transporter needed to reach 50% chloride transport after 5 min, was estimated to be 1 μM, suggesting that this compound is indeed very active. 99This study also established that (C 6 F 5 ) 3 Sb (3) does not behave ideally, as it also destabilizes the vesicles.A subsequent study also indicated that this molecule is not water stable. 100These stability limitations do not seem to affect (3,4,5-C 6 F 3 H 2 ) 3 Sb ( 8) and (2,4,6-C 6 F 3 H 2 ) 3 Sb (61), which were also evaluated using the above-mentioned HPTS assay. 100Remarkably, while 61 showed an activity that is moderately improved when compared to that of 4 (EC 50 = 0.27 ± 0.02 μM for 61 vs 1 μM for 4), compound 8 displays a much higher activity as indicated by its EC 50 value of 2.6 ± 0.8 nM, which is 3 orders of magnitude lower than that of 4. While no explanation for this significant increase between 4 and 8 is provided, it was suggested that 61 is penalized by the ortho-fluorine substituents that may hinder the anion binding site or donate to the antimony atom, thereby reducing its Lewis acidity or pnictogen bond donor ability.
Concomitant with the above efforts, we investigated the anion transport properties of stibonium cations. 101These studies, which were encouraged by the known ability of triarylphosphonium cations to permeate through biological   membranes, 102 first focused on tetraarylstibonium cations that were already known to promote anion phase transfer while also serving as water compatible anion sensors.13a,60,90,92 We first evaluated [36] + , [56] + , [57] + , and [62] + using a fluoride ion selective electrode (ISE) and EYPC LUVs loaded with KF (Figure 30). 101We found all four compounds to be potent transporters, either via an F − /OH − antiport mechanism or via KF efflux when administered in the presence of valinomycin as a potassium ion transporter (Figure 30).While all four cations are potent transporters, we observed that the activities of (POPC) vesicles by Gale and co-workers. 103We will also note that these values are close to those measured for phosphonium boranes, 104 which we have also used as fluoride anion transporters.The observation of fluoride transport in the absence of valinomycin provided initial evidence that these stibonium cations may also transport hydroxide anions.We confirmed this possibility using [36] + , which was deployed in an HPTS assay using EYPC vesicles.This conclusion is reinforced by a recent study employing a lanthanide-based fluoride sensor immobilized inside POPC vesicles to assess fluoride influx. 105Using different fluorescence assays, this study concluded that, under the experimental conditions, [56] + is indeed a potent hydroxide anion transporter.It follows that [56] + may also induce acidification of the vesicle interior and thus formation of HF (pK a = 3.17), which can diffuse spontaneously through the membrane.This possibility serves as a reminder that studying the transport of basic anions such as fluoride is inherently complicated.
The fluoride transport properties of stibonium cations do not seem to be affected by the intramolecular coordination of a donor group to the antimony center.Such is the case for the aforementioned [38] + whose gas phase FIA is dampened by intramolecular coordination of a phosphine oxide compared to the parent [36] + (125.0 kcal•mol −1 and 133.0 kcal•mol −1 , respectively). 70Nevertheless, this cation is a very effective transporter, as evidenced by its ability to transport fluoride across the membranes of POPC LUVs (Figure 30).The EC 50 value of 0.24 ± 0.03 mol % shows that its potency is on par with [56] + , leading us to propose that the high lipophilicity of [38] + (log K ow = 7.20) plays a determining role in its performance.These results demonstrate that the transport properties of these stibonium cations survive the introduction of a Lewis basic ligand, providing another dimension along which the composition, properties, and possible conjugability of the transporter could be adjusted.
Along similar lines, we observed that the o-phenylthioetherstibonium cation [40] + is also a very potent anion transporter, as indicated by an assay that employed POPC LUVs loaded with KCl (Figure 31). 71Indeed, the EC 50 of this compound was found to be 0.63 ± 0.03 mol %.The elevated activity of this transporter is correlated to the Lewis acidity of the antimony center as well as the overall lipophilicity of the structure (log K ow = 7.68), which promotes recruitment of the transporter to the hydrophobic part of the membrane.As part of this study, we also assayed its corresponding sulfoniumstibonium dication [42] 2+ .Even though it is substantially more chloridophilic than [40] + , [42] 2+ is a significantly less effective  transporter.The lower performance of this species is assigned to its decreased lipophilicity (log K ow = 1.26), which may limit its ability to partition into the membrane.The contrasting properties of these two compounds led us to hypothesize that the [40] + / [42] 2+ pair could serve as the basis for a stimulusresponsive transport system.With this idea in mind, we decided to investigate whether [42] 2+ undergoes reduction of the sulfonium center and thus convert into the much more active [40] + .This possibility was tested using glutathione (GSH), which we first verified cleanly reduces [42] 2+ into [40] + , in aqueous solutions (D 2 O:d 6 -DMSO, 7.9:2.1 (v/v), pH 7.6, 300 mM sodium phosphate) by 1 H NMR spectroscopy.We also confirmed that the addition of GSH to a solution of KCl-loaded POPC vesicles pretreated with [42] 2+ followed by incubation leads to increased chloride efflux upon administration of valinomycin, as a result of the in situ conversion of [42] 2+ into the more active [40] + .Increasing the concentration of GSH showed a positive correlation with the extent of transport activation, adding further credence to our interpretation (Figure 31). 71inally, with the view of testing stibonium-based anion transporters in biological settings, we decided to test their ability to accelerate the known toxicity of fluoride toward human red blood cells (RBCs), causing oxidative stress and hemolysis when in the presence of fluoride. 106Using this toxic side effect as a marker of anion transport, we compared the rate of hemolysis of a sample of RBCs in fluoridated media (100 μM) and [56] + (5 μM). 101The extent of hemolysis was monitored over time, reaching 48% after 8 h.By comparison, incubation of the RBCs in fluoridated media alone induced only 17% hemolysis after 8 h, while the transporter [56] + in the absence of fluoride was found to be nontoxic.The accelerated fluoride-induced toxicity observed in the presence of [56] + suggests that the stibonium cation facilitates transport of the fluoride anions inside the RBCs (Figure 32).

■ CONCLUSIONS AND OUTLOOK
The chemistry of organoantimony derivatives has long been limited to structural studies.Over the course of several decades, these studies have revealed the Lewis acidic tendencies that antimony displays especially when bearing electron-withdrawing ligands or when in the pentavalent state.Recent efforts aimed at reclassifying the natural Lewis acidity of antimony derivatives as its ability to form pnictogens bond, or in other words dative bonds with donors have continued to mostly focus on deciphering the structure of the resulting adducts and the nature of the bond connecting the antimony center to the donor.Strikingly, until about 15 years ago, very few efforts had been made to translate the natural Lewis acidity of organoantimony species into a functional context.Our work from the past 12 years, and the contributions of other groups, has begun to change this state of affairs with the demonstration that the Lewis acidity of these main group derivatives can be purposefully exploited for application in anion sensing, anion binding catalysis, and anion transport.The results presented in this Perspective, which focus on our contribution to this area, underscore several important facets, including the ease of synthesis of Lewis acidic organoantimony compounds, their stability toward ambient and aqueous conditions, and the facile tunability of their Lewis acidity, that enable applications in aqueous chemistry where the antimony center is not irreversibly neutralized by water.These group 15 Lewis acids, whose tunability is reminiscent of that of boron-based systems, surpass their group 13 analogs in such conditions .
Thus, the results presented in this Perspective serve as a prelude for antimony's bright future, specifically in the context of anion binding chemistry.The diverse structural variations that can be implemented to augment anion binding and the simple strategies that can be used to encode photophysical responses provide initial evidence of future possibilities in the field of anion sensing.The same is true in the domain of anion transport chemistry, where stibonium cations have found a new niche as potent carriers of biologically relevant anions across phospholipid membranes.We note though that a deeper understanding of the anion selectivity of these transporters is imperative given that a target anion will constantly compete with hydroxide binding at biological pH values.Attention should also be directed toward expanding the structural breadth of these compounds such that they can toggle their Lewis acidic properties.Clearly the lipophilic character of the transporter influences its anionophoric activity, but the influence of other biological triggers on the transporter structure remains to be determined.
Finally, returning to the fundamental aspects that underpin this chemistry, we will note that while extremely useful the σ hole concept is limited in terms of its ability to predict how anion binding to an antimony center may alter the electronic features of the entire platform.Such a limitation is made particularly obvious by our work on anion sensing, where orbital and electronic structure analyses are necessary to explain the various colorimetric and fluorescence responses that we have observed.Thus, we recommend that the σ hole and σ* orbital models always be paired when considering such chemistry.

Figure 1 .
Figure 1.Illustration of the structural and energetic components of chloride binding by Pn(III) centers investigated by Bickelhaupt and co-workers.

Figure 2 .
Figure 2. Schematic representations of the "σ hole" (left) and the σ* orbital of a trivalent pnictogen.The σ* orbital is presented in a simplified fashion, with the second representation only keeping the accepting lobe.A similar picture could be drawn for a pentavalent pnictogen.R ewg is an electron-withdrawing group.

Figure 5 .
Figure 5. Deepening of the σ hole and stabilization of the σ* orbital resulting from oxidation of an Sb(III) species to an Sb(V) species.

Figure 9 .
Figure 9. Top: Oxidation of distibine 29 to distiborane 30 following treatment with two equiv of o-chloranil.Bottom: Contour plot of the LUMO (isovalue: 0.05) of 30 showing the projection of the σ*(Sb− C/O) orbitals into the binding pocket.

19 F
46-F upon treatment with [TAS][Me 3 SiF 2 ] (TASF).While the NMR chemical shift of this complex (−140.1 ppm) is in the range expected for a triarylfluoroborate species, 4c its crystal structure shows that the fluoride anion bridges the two Lewis acids, resulting in a B−F bond of 1.521(4) Å and an Sb−F bond of 2.450(2) Å.A competition experiment involving [o-(Ph 2 MeP)(Mes 2 B−F)C 6 H 4 ] (47-F),

Figure 18 .
Figure 18.Top: Conversion of 1-chloroisochroman to methyl 2-(isochroman-1-yl)acetate via C−Cl activation by stibines 3−5.Bottom: Working model of the catalysis facilitated by 3 depicting the heterolytic dissociation of the C−Br bond at the Lewis acidic stibine center.

Figure 19 .
Figure 19.Top: Conversion of diphenylbromomethane to Nbenzyhydrylacetamide in d 3 -MeCN via C−Br activation by stibine 12 and catecholatostiborane 25.Bottom: Working model of the catalysis facilitated by 25 depicting the heterolytic dissociation of the C−Br bond at the Lewis acidic catecholatostiborane center.

Figure 21 .
Figure 21.Top left: Chloride anion binding by λ 3 -stibaindoles 51 and 52 in MeCN.Top right: Idealized rendition of the origin of the response via the chloride-induced disruption of the σ*/π* conjugation.Bottom: Solid-state structure of [51-Cl] − .

Figure 22 .
Figure 22.Top: Synthesis of 1λ 5 -stibaindoles 53 and 54 was achieved by oxidation of 52 with ortho-quinones.Bottom left: Idealized rendition of the origin of the response via fluoride-binding-induced disruption of the σ*/π* conjugation at the LUMO.Bottom right: UV−vis spectra showing the response of 53 to fluoride binding in CHCl 3 .

Figure 23 .
Figure 23.Left: Fluoride binding behavior of by 55 in a THF:water (7:3 (v/v)) solution and colorimetric response to fluoride complexation in CH 2 Cl 2 .Right: UV−vis changes induced by fluoride complexation to 55 in CH 2 Cl 2 .

Figure 25 .
Figure 25.Schematic diagram of the geometry-induced orbital reordering responsible for the emissive properties of [56] + .

Figure 27 .
Figure 27.Left: Fluoride binding by [60] + in MeCN, with an idealized rendition of the charge-transfer disruption upon fluoride binding.Right: Fluorescence response to fluoride binding in MeCN.Figure 28.Antimony-based Lewis acids transporting anions across a phospholipid bilayer membrane via a carrier-type mechanism.

Figure 28 .
Figure 27.Left: Fluoride binding by [60] + in MeCN, with an idealized rendition of the charge-transfer disruption upon fluoride binding.Right: Fluorescence response to fluoride binding in MeCN.Figure 28.Antimony-based Lewis acids transporting anions across a phospholipid bilayer membrane via a carrier-type mechanism.

Figure 29 .
Figure 29.Simplified illustration of the HPTS assay as employed by Matile using EYPC LUVs depicting Cl − /OH − antiport upon administration of a base pulse.The structures of stibines 3, 4, 8, and 61 used as transporters are also shown.
[56] + and[57] + far exceed those of[36] + and[62] + .These observations led us to propose that the lipophilic character of the stibonium structure assists in partitioning the transporter into the membrane, thus leading to more effective transport.This is supported by computed n-octanol/water partition coefficient calculations (log K ow ), which showed that[56] + and [57] + were at least an order of magnitude more lipophilic (log K ow values of 5.85 and 6.26, respectively) than the lower performing[36] + and [62] + (log K ow values of 4.19 and 5.09, respectively).Transport data collected with [56] + and [57] + in the presence of valinomycin afforded EC 50 values at 270 s of 0.41 ± 0.05 and 0.57 ± 0.07 mol %, respectively.These EC 50 values indicate that [56] + and [57] + rival the transport properties of strapped calix[4]pyrroles investigated via fluoride ISE in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine